Please remember I'm a user too. I taught myself just like most.
Support staff on email would have probably asked the similar questions

Oblivian
Very sorry if you thought I was suggesting you were unhelpful, having been on the receiving end of users questions I fully understand you have to know how they got to where they are before you can help.

ABDC et al
Thanks for all your help.
I reran the installation...again, and this time got to the sign up page.
It reported my email and feeder code didn't match. ( I have had several different feeder numbers whilst trying to get back on-line.)
By coincidence at exactly this time I received an email from FR24, saying they missed me, and did I have a problem as I hadn't fed for a while?
So my attempts are on hold whilst they sort out what feeder code they think I should use.
Once that's sorted I'll be back using ABCD's sign on and wiedehopf's IP6 blockers.
I am not dead, just sleeping!
D

two steps forward one step back

Thanks folk, I think I'm getting closer.
FR have corrected the email they had on file.
I followed ABCD's excellent guide literally "to the letter" and using "fr24feed --signup" I have got through ALMOST all of the sign up process.
so
Imagine the joy seeing:
Congratulations! You are now registered and ready to share ADS-D data with Flightradar24.
+ Your radar id is T-EGHH99, please include it in all email communication with us.
+ Please make sure to start sharing data within one month from now as otherwise your ID/KEY will be deleted.
Thank you for supporting Flightradar24! We hope that you will enjoy our Premium services that will be available to you when you become an active feeder.
To start sending data now please execute:
sudo systemctl start fr24feed

BUT
Imagine the disappointment at seeing:

Saving settings to /etc/fr24feed.ini...ERROR

Full of hope I entered :
sudo systemctl restart fr24feed
with no effect and fr24feed -status gave [Fail] FR24 Feeder/Decoder Process ........ failed!
In the best traditions of computing I have tried rebooting Pi and executing the restart but there is no change.

I wonder why it couldn't save the ini file? (Yes I have checked the write protect switch on the SD card!)

ABCD's STEP-7: Checks

Status in PuTTY : [FAIL] FR24 Feeder/Decoder Process ... failed!
Status in Browser: and Settings 192.168.0.23 refused to connect.
check that dump1090 is working http://192.168.0.23/dump1090/gmap.html
Gave a map, two clocks but 0 aircraft and a yellow box with:
Problem fetching data from dump1090.
AJAX call failed (error: Not Found). Maybe dump1090 is no longer running?
The displayed map data will be out of date.